1. Field of the Invention
The present invention relates to non-directional antennas used for broadband communication.
2. Description of the Related Art
In recent years, UWB (ultra wideband) communication, which is a communication technology that uses an extremely wide frequency band, that can coexist with existing wireless technology and that allows high-speed broadband wireless communication, has garnered considerable attention. UWB communication uses a frequency band of 3.1 GHz to 10.6 GHz for short pulses of only about 1 ns duration. It enables high-speed communication by exclusively using an extremely wide frequency band of several GHz width.
On the other hand, the distance over which communication is possible in UWB communication is short. Therefore, it has been proposed to utilize UWB in wireless interfaces to perform data transfer between computers and peripheral devices.
As the antennas used for UWB communication, there are biconical antennas. The structure of such biconical antennas is disclosed in JP 2001-185942A and JP H9-8550A, for example.
As shown in FIG. 11, in an ordinary biconical antenna 40, frustum-shaped metal members 42 and 44 are placed in opposition to each other with a gap G between them. One of these metal members is a feeder portion 42 and the other is a ground portion 44. The feeder portion 42 is connected to the center conductor 30 of a coaxial cable 34, and the ground portion 44 is connected to the shield conductor 32 of the coaxial cable 34. The emission and reception of electromagnetic waves is carried out with the lateral surface (inclined surface) of the feeder portion 42.
When this biconical antenna is used for data transfer between a computer and peripheral devices by UWB communication, then the biconical antenna needs to be attached to the computer, and in particular when attaching it to a notebook computer, there is a need for making the biconical antenna small.
However, as far as the size of the biconical antenna is concerned, the length of the frustum-shaped lead line in the biconical antenna disclosed in JP H9-8550A is 25 cm, which is too large to attach it to a notebook computer. There are no particular statements regarding size in JP 2001-185942A. Furthermore, in JP 2001-185942A and JP H9-8550A, there are no particular statements concerning making the biconical antenna smaller and using it as a wireless interface for computers. Due to their size, it would be difficult to use the conventional biconical antennas disclosed in JP 2001-185942A and JP H9-8550A as a wireless interface for computers. Moreover, as mentioned above, the frequency region for UWB communication is the microwave frequency region. Therefore, a considerable precision is required when manufacturing the antenna 40. Also, if there are discrepancies in shape or dimensions of the antenna 40 during the manufacturing the antenna, or if there are scratches or the like on the surface of the antenna 40, then the antenna characteristics will change. Therefore, an extremely high precision is required in the manufacturing process of the biconical antenna 40 when trimming the frustum-shaped metal or when assembling the biconical antenna 40.